I lost about 20 pounds; mostly in my hip and thighs. I want to put some weight/muscle mass back on in that area. I eat a lot of food and go for walks but I don't gain any weight there. What can I do to gain weight there even if I don't have access to the gym?

It is harder to gain a lot of muscle by just walking. You should give pilates a try or try other leg work outs. There are a lot of really good workouts on YouTube that you can do that will build muscle.
YouTube - leg workout (http://www.youtube.com/results?search_query=leg+workout&page=1)